Honeycomb filter and exhaust gas purifying apparatus

ABSTRACT

A honeycomb filter includes a pillar-shaped honeycomb fired body having cells longitudinally disposed with a cell wall and having first and second end faces on gas inlet and outlet sides, respectively. A catalyst supporting layer is formed in a catalyst-supporting-layer area covering at least about 25% and at most about 90% of an overall length of the fired body and that abuts the first end face. Substantially no catalyst supporting layer is formed in a non-catalyst-supporting-layer area covering about 10% of the overall length that abuts the second end face. A thermal conductivity of the non-catalyst-supporting area is larger than that of the catalyst-supporting-layer area, where gas permeability coefficient k1 (μm 2 ) of a cell wall in the non-catalyst-supporting-layer area and gas permeability coefficient k2 (μm 2 ) of a cell wall in the catalyst-supporting-layer area satisfy inequalities, (k1−k2)≦about 0.5 and about 1.0≦k1≦about 1.5.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to PCTApplication No. PCT/JP2007/057303, filed on Mar. 30, 2007, the contentsof which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a honeycomb filter and an exhaust gaspurifying system.

2. Discussion of the Background

In recent years, particulate matter (hereinafter, also referred to as“PM”) such as soot contained in exhaust gases discharged from internalcombustion engines of vehicles such as buses and trucks, constructionmachines and the like have raised serious problems as contaminantsharmful to the environment and the human body. For this reason, varioushoneycomb filters, which use a honeycomb structured body made of porousceramics, have been proposed as filters that capture PM in exhaust gasesand purify the exhaust gases.

In a honeycomb filter of this kind, a catalyst used for purifying and/orconverting exhaust gases may be supported thereon, and in this case, acatalyst supporting layer is formed in an area on which the catalyst isto be supported, so that the catalyst is supported on the catalystsupporting layer.

JP-A 2003-154223 has disclosed a honeycomb filter made from siliconcarbide, in which a higher amount of catalyst is supported on the sidethat allows exhaust gases to flow in (gas inlet side) and a lesseramount of catalyst is supported on the side that allows exhaust gases toflow out (gas outlet side), or a catalyst is supported only on the gasinlet side and substantially no catalyst is supported on the gas outletside; and an exhaust gas purifying system in which the honeycomb filterof this kind is placed in an exhaust gas passage.

JP-A 2003-161138 has disclosed a honeycomb filter that is designed tomake the amount of supported catalyst successively smaller step by stepor continuously, from the gas inlet side toward the gas outlet side ofthe honeycomb filter.

The contents of JP-A 2003-154223 and JP-A 2003-161138 are incorporatedherein by reference in their entirety.

SUMMARY OF THE INVENTION

A honeycomb filter of the present invention includes a pillar-shapedhoneycomb fired body having a large number of cells longitudinallydisposed in parallel with one another with a cell wall therebetween,with either one end of each of the cells being sealed. The honeycombfired body has a first end face on a gas inlet side and a second endface on a gas outlet side such that the honeycomb filter is configuredto allow gases to flow in through the gas inlet side and to flow outfrom the gas outlet side. A catalyst supporting layer is formed in acatalyst-supporting-layer area that covers at least about 25% and atmost about 90% of an overall length of the honeycomb fired body and thatabuts the first end face on the gas inlet side. Substantially nocatalyst supporting layer is formed in a non-catalyst-supporting-layerarea that covers about 10% of the overall length of the honeycomb firedbody and that abuts the second end face on the a gas outlet side. Athermal conductivity of the non-catalyst-supporting area is larger thana thermal conductivity of the catalyst-supporting-layer area, where gaspermeability coefficients k1 (μm²) and k2 (μm²) satisfy the followinginequalities (1) and (2):(k1−k2)≦about 0.5  (1), andabout 1.0≦k1≦about 1.5  (2),in which the gas permeability coefficient k1 is a gas permeabilitycoefficient of a cell wall in the non-catalyst-supporting-layer area inthe honeycomb filter, and the gas permeability coefficient k2 is a gaspermeability coefficient of a cell wall in the catalyst-supporting-layerarea in the honeycomb filter.

An exhaust gas purifying apparatus of the present invention includes: ahoneycomb filter; a casing covering an outside of the honeycomb filter;and a holding sealing material interposed between the honeycomb filterand the casing. The honeycomb filter includes a pillar-shaped honeycombfired body having a large number of cells longitudinally disposed inparallel with one another with a cell wall therebetween, with either oneend of each of the cells being sealed. The honeycomb fired body has afirst end face on a gas inlet side and a second end face on a gas outletside such that the honeycomb filter is configured to allow gases to flowin through the gas inlet side and to flow out from the gas outlet side.A catalyst supporting layer is formed in a catalyst-supporting-layerarea that covers at least about 25% and at most about 90% of an overalllength of the honeycomb fired body and that abuts the first end face onthe gas inlet side. Substantially no catalyst supporting layer is formedin a non-catalyst-supporting-layer area that covers about 10% of theoverall length of the honeycomb fired body and that abuts the second endface on the a gas outlet side. A thermal conductivity of thenon-catalyst-supporting area is larger than a thermal conductivity ofthe catalyst-supporting-layer area, where gas permeability coefficientsk1 (μm²) and k2 (μm²) satisfy the following inequalities (1) and (2):(k1−k2)≦about 0.5  (1), andabout 1.0≦k1≦about 1.5  (2),in which the gas permeability coefficient k1 is a gas permeabilitycoefficient of a cell wall in the non-catalyst-supporting-layer area inthe honeycomb filter, and the gas permeability coefficient k2 is a gaspermeability coefficient of a cell wall in the catalyst-supporting-layerarea in the honeycomb filter.

BRIEF DESCRIPTIONS OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a graph that shows an example of the result of plotting Q onthe horizontal axis and ΔP/Q on the longitudinal axis;

FIG. 2 is a perspective view that schematically shows one example of anembodiment of a honeycomb filter of the present invention;

FIG. 3A is a perspective view that schematically shows one example of anembodiment of a honeycomb fired body forming a honeycomb filter of thepresent invention, and FIG. 3B is a cross-sectional view taken along theline A-A of the FIG. 3A;

FIGS. 4A to 4D are cross-sectional views, each of which schematicallyshows one example of an embodiment of a honeycomb fired body in which acatalyst supporting layer is formed in a predetermined area;

FIG. 5 is an explanatory view that shows a pressure loss measuringapparatus;

FIG. 6 is a cross-sectional view that shows an exhaust gas purifyingapparatus used upon measuring a regeneration limit value;

FIG. 7 is a magnified cross-sectional view that schematically shows amethod for measuring a thickness of PM on a cut plane of a honeycombfilter;

FIG. 8 is an explanatory view that shows a capturing efficiencymeasuring apparatus;

FIG. 9 is a graph that shows a relationship among a difference in thegas permeability coefficients on the gas inlet side and the gas outletside, the regeneration limit value, and a PM thickness difference inrespective Examples and Comparative Examples; and

FIG. 10 is a graph that shows a relationship between a formation rangeof a catalyst supporting layer and a regeneration limit value.

DESCRIPTION OF THE EMBODIMENTS

The embodiments will now be described with reference to the accompanyingdrawings, wherein like reference numerals designate corresponding oridentical elements throughout the various drawings.

In a normal honeycomb filter, since the temperature in the honeycombfilter on the gas outlet side tends to become higher than thetemperature in the honeycomb filter on the gas inlet side upon passageof high temperature exhaust gases, PM is sufficiently burned even whenthe amount of catalyst supported on the gas outlet side of the honeycombfilter is small. The honeycomb filter disclosed in JP-A 2003-154223 andJP-A 2003-161138 has been manufactured considering the phenomenon.

In the honeycomb filters disclosed in JP-A 2003-154223 and JP-A2003-161138, it may be easier to reduce the amount of catalyst to besupported on the gas outlet side, and consequently to cut productioncosts.

Moreover, by reducing the amount of catalyst to be supported, theinitial pressure loss may be lowered more easily.

It is considered that, in general, the honeycomb filter should beprovided with a high regeneration limit value (the maximum value ofamount of captured PM which would not cause any cracks in the filtereven upon burning captured PM). This is because frequent regenerationprocesses are required in an exhaust gas purifying system using ahoneycomb filter with a low regeneration limit value, which leads to aproblem of lowering fuel economy of an internal combustion engine.

However, there has been no consideration on the regeneration limit valuein the conventional honeycomb filter (JP-A 2003-154223 and JP-A2003-161138), so that the conventional honeycomb filter presumably needsimprovement in terms of the regeneration limit value.

In accordance with embodiments of the present invention, a honeycombfilter having a higher regeneration limit value is provided. Thefollowing was found.

That is, by setting the area covering at least 10% of the overall lengthof the honeycomb filter from the end face on the gas outlet side of thehoneycomb filter as an area without a catalyst supporting layer beingformed therein and setting the thermal conductivity of this areacovering at least 10% of the overall length larger than the thermalconductivity of the area of the honeycomb filter on which a catalystsupporting layer is formed, heat radiation in the end face neighborhoodon the gas outlet side surely progress. In this case, the temperaturerise on the gas outlet side is suppressed, and therefore, a thermalimpact caused by the temperature difference between the gas inlet sideand the gas outlet side of the honeycomb filter tends not to begenerated, thereby more easily achieving a high regeneration limit valuein the honeycomb filter.

Moreover, in the case of supporting a catalyst on the honeycomb filter,since a reaction of gases having flowed in generates heat in the area inwhich the catalyst is supported, the calorific value in the area inwhich the catalyst is supported tends to become greater than that of thearea in which the catalyst is not supported. Here, when the area inwhich the catalyst is supported is too narrow, a large amount of heat ispresumably generated in the narrow area. Further, since the area inwhich the catalyst supporting layer is formed has a lower thermalconductivity compared to the area in which the catalyst supporting layeris not formed, it is assumed that the area is in a state that hardlycauses heat radiation.

For this reason, when the catalyst supporting layer having a catalystsupported thereon is formed in a narrow area, the temperature differencebetween the corresponding area and the other areas becomes very large,with the result that a greater thermal impact is probably applied ontothe honeycomb filter.

To facilitate prevention of a large amount of heat from being generatedwithin a narrow area and consequent prevention of a great thermal impactfrom being applied onto the honeycomb filter, it was found that, byforming the catalyst supporting layer in the area covering about 25% ormore of the overall length of the honeycomb filter out of the areacovering about 90% of the overall length of the honeycomb filter fromthe end face on the gas inlet side, it may be easier to achieve a highregeneration limit value in the honeycomb filter.

Furthermore, with respect to the honeycomb filter with the catalystsupporting layer being formed only in the predetermined area asdescribed above, a state of the honeycomb filter after capturing PM wasobserved. Then, surprisingly, a phenomenon has been confirmed in whichmore PM had been captured on the gas outlet side (area in whichsubstantially no catalyst supporting layer is formed) compared to thoseon the gas inlet side (area in which the catalyst supporting layer isformed).

As indicated by the results of the observation, it is considered thatcarrying out a regeneration process on a honeycomb filter with more PMcaptured on the gas outlet side of the honeycomb filter makes thetemperature on the gas outlet side higher than the temperature on thegas inlet side; therefore, the temperature difference between the gasinlet side and the gas outlet side of the honeycomb filter becomesgreater, with the result that the thermal impact to be applied onto thehoneycomb filter becomes greater and cracks are easily generated.

From this point of view, it is presumed that more PM being captured onthe gas outlet side of the honeycomb filter causes a reduction in theregeneration limit value of the honeycomb filter.

Further, the reason why more PM is captured on the gas outlet side ofthe honeycomb filter was studied. In these efforts, when a gaspermeability coefficient was measured on each of cell walls in the areaon the gas inlet side and the area on the gas outlet side, a remarkabledifference was confirmed between the gas permeability coefficients ofthe cell walls in the two areas, and the gas permeability coefficient ofthe cell wall on the gas inlet side was considerably smaller than thegas permeability coefficient of the cell wall on the gas outlet side.

Here, the gas permeability coefficient is a parameter that indicateseasiness of gases passing through the cell wall of a honeycomb filter.As the gas permeability coefficient becomes higher, the gases areallowed to pass through the cell wall more easily, and therefore, it ispresumed that the cell wall on the gas inlet side having a low gaspermeability coefficient makes exhaust gases hard to pass through thecell wall, and that consequently, most of the exhaust gases are made topass through a cell wall on the gas outlet side.

Here, since a larger amount of PM is captured on a cell wall throughwhich more exhaust gases have passed, it was presumed that thedifference in the gas permeability coefficients of the gas inlet sideand the gas outlet side of a honeycomb filter is the reason why a largeramount of PM is captured on the gas outlet side of a honeycomb filter.

Based upon these, it was found that, in order to increase theregeneration limit value of the honeycomb filter, the honeycomb filtershould have a structure in which PM is captured as uniformly as possiblefrom the gas inlet side toward the gas outlet side of the honeycombfilter by designing the honeycomb filter to have smaller difference inthe gas permeability coefficients of the cell walls between the gasinlet side and the gas out let side.

Namely, the honeycomb filter according to the embodiments of the presentinvention is a honeycomb filter including a pillar-shaped honeycombfired body having a large number of cells longitudinally disposed inparallel with one another with a cell wall therebetween, with either oneend of each of the cells being sealed. The honeycomb fired body has afirst end face on a gas inlet side and a second end face on a gas outletside such that the honeycomb filter is configured to allow gases to flowin through the gas inlet side and to flow out from the gas outlet side.A catalyst supporting layer is formed in a catalyst-supporting-layerarea that covers at least about 25% and at most about 90% of an overalllength of the honeycomb fired body and that abuts the first end face onthe gas inlet side, and substantially no catalyst supporting layer isformed in a non-catalyst-supporting-layer area that covers about 10% ofthe overall length of the honeycomb fired body and that abuts the secondend face on the a gas outlet side.

A thermal conductivity of the non-catalyst-supporting area is largerthan a thermal conductivity of the catalyst-supporting-layer area, wheregas permeability coefficients k1 (μm²) and k2 (μm²) satisfy thefollowing inequalities (1) and (2):(k1−k2)≦about 0.5  (1), andabout 1.0≦k1≦about 1.5  (2),in which the gas permeability coefficient k1 is a gas permeabilitycoefficient of a cell wall in the non-catalyst-supporting-layer area inthe honeycomb filter, and the gas permeability coefficient k2 is a gaspermeability coefficient of a cell wall in the catalyst-supporting-layerarea in the honeycomb filter.

In the honeycomb filter according to the embodiments of the presentinvention, the gas permeability coefficients of the cell walls of thetwo areas are adjusted so that the gas permeability coefficients k1 andk2 satisfy the relationship of (k1−k2)≦about 0.5.

Between the gas permeability coefficient k and the pressure loss ΔP(kPa), the relationship indicated by the following equations (3) and (4)is held.

$\begin{matrix}{{\Delta\; P} = {{\frac{\mu\; Q}{2V_{trap}}{( {a + w_{s}} )^{2}\lbrack {\frac{w_{s}}{ka} + \frac{8{FL}}{3a^{4}}} \rbrack}} + {\frac{2{\zeta\rho}\;{Q^{2}( {a + w_{s}} )}^{4}}{V_{trap}^{2}a^{2}}( \frac{L}{a} )^{2}}}} & (3)\end{matrix}$

Here, Q represents a gas flow rate (m³/s). μ represents a gas kinematicviscosity. Vtrap represents an effective honeycomb structured bodyvolume (m³). “a” represents a cell width (m). Ws represents a cell wallthickness (m). L represents an outer diameter (m) of the honeycombstructured body. ζ, which is referred to as a compression/expansioninertia loss coefficient, represents a parameter derived from adiscontinuous change in the gas permeable cross sectional area on eachend face of the honeycomb structured body, and is mainly determined bythe aperture ratio and shape of each end face of the honeycombstructured body. ρ represents a gas density (kg/m³). F represents aconstant value (28.454). Out of these, Vtrap, a, Ws, L, and ζ aredetermined by the shape of the honeycomb structured body, and μ and ρare uniquely determined by the temperature and the kind of a gas.

By dividing the both sides of equation (3) by Q and sorting the result,a linear equation of Q to ΔP/Q as indicated by the following equation(4) is obtained.

$\begin{matrix}{\frac{\Delta\; P}{Q} = {{\frac{2{{\zeta\rho}( {a + w_{s}} )}^{4}}{V_{trap}^{2}a^{2}}{( \frac{L}{a} )^{2} \cdot Q}} + {\frac{\mu\; w_{s}}{2{aV}_{trap}}{( {a + w_{s}} )^{2} \cdot \frac{1}{k}}} + {\frac{\mu}{2V_{trap}}( {a + w_{s}} )^{2}\frac{8{FL}}{3a^{4}}}}} & (4)\end{matrix}$

The gas permeability coefficient k can be calculated by using theequation (4).

FIG. 1 is one example of a graph obtained by plotting Q on thehorizontal axis and ΔP/Q on the longitudinal axis.

In order to calculate the gas permeability coefficient k, by varying thegas flow rate Q to several values, the pressure loss ΔP is measured, andQ is plotted on the horizontal axis and ΔP/Q is plotted on thelongitudinal axis in a graph as shown in FIG. 1. Here, these plottedvalues virtually form a straight line, so that the relationship betweenQ and ΔP/Q can be represented by an approximate straight line as shownin FIG. 1. Moreover, the value at the intercept can be obtained from avalue at an intersection between the approximate straight line and thelongitudinal axis.

In the measurements, since variables other than the gas permeabilitycoefficient k in equation (4) are known quantities determined by themeasuring conditions and the structure of the honeycomb structured body,equation (4) is represented as the following equation (5) by usingconstant values A, C₁, and C₂.

$\begin{matrix}{\frac{\Delta\; P}{Q} = {{AQ} + \frac{C_{1}}{k} + C_{2}}} & (5)\end{matrix}$

Namely, the value at the intercept obtained from the plotted line ofFIG. 1 is represented by (C₁/k+C₂), and based upon this, the gaspermeability coefficient k can be calculated.

In the honeycomb filter according to the embodiments of the presentinvention, a difference between the gas permeability coefficient of thearea (gas inlet side) in which the catalyst supporting layer is formedand that of the area (gas outlet side) in which substantially nocatalyst supporting layer is formed becomes about 0.5 μm² or less byadjusting the gas permeability coefficients of the cell walls of the twoareas.

In the honeycomb filter according to the embodiments of the presentinvention, since the difference in the gas permeability coefficients iscontrolled within the above-mentioned range, it may be easier to reducea difference between easiness of the exhaust gases passing through thecell wall on the gas inlet side and the easiness of the exhaust gasespassing through the cell wall on the gas outlet side.

Thus, out of exhaust gases allowed to pass through the cell wall, theproportion of those exhaust gases passing through the area in which thecatalyst supporting layer is formed, that is, the cell wall on the gasinlet side, is more likely to be increased; therefore, it allows thecell wall on the gas inlet side to capture more PM.

As a result, the amount of PM to be captured on the cell wall on the gasoutlet side tends to be relatively reduced, so that the temperaturedifference between the gas inlet side and the gas outlet side uponburning PM tends to become smaller. Therefore, cracks hardly occur atthe time of regeneration process, and consequently, it may be easier toprovide a honeycomb filter having a high regeneration limit value.

Here, in a case where the gas permeability coefficient k1 of the cellwall in the area in which substantially no catalyst supporting layer isformed is about 1.5 μm² or less, this state is not desirable because thecapturing efficiency of PM is less likely to be lowered. Moreover, in acase where k1 is about 1.0 μm² or more, this state is not desirablebecause the pressure loss does not become too high.

Moreover, in the honeycomb filter according to the embodiments of thepresent invention, since substantially no catalyst supporting layer isformed in an area covering about 10% of the overall length of thehoneycomb filter from the end face on the gas outlet side, the thermalconductivity of the area in which substantially no catalyst supportinglayer is formed is larger than the thermal conductivity of the area inwhich the catalyst supporting layer is formed in the honeycomb filter.

By setting the area covering about 10% of the overall length of thehoneycomb filter from the end face on the gas outlet side as an areamade from a member having a higher thermal conductivity, it may beeasier to accelerate heat radiation from the end face neighborhood onthe gas outlet side. Consequently, since the temperature rise on the gasoutlet side of the honeycomb filter may be prevented more easily, athermal impact caused by a temperature difference between the gas inletside and the gas outlet side of the honeycomb filter hardly occurs,making it easier to provide a honeycomb filter having a highregeneration limit value.

Moreover, in a case where a catalyst is supported, heat is generated dueto reaction of the gas, and thus the heating value of the area on whichthe catalyst is supported tends to become higher than the other area onwhich substantially no catalyst is supported. Furthermore, since thearea in which the catalyst supporting layer is formed has a lowerthermal conductivity than the area on which substantially no catalystsupporting layer is formed, heat radiation hardly occurs therein. Forthis reason, it is presumed that in a case where the area on which thecatalyst is supported is too narrow, a large quantity of heat isgenerated in the narrow area to cause a larger temperature differencebetween the area in which the catalyst is supported on the catalystsupporting layer and the other area in which substantially no catalystsupporting layer is formed, resulting in a larger thermal impact to beapplied to the honeycomb filter.

In contrast, when the catalyst supporting layer is formed in an areacovering about 25% or more of the overall length of the honeycomb filterfrom the end face on the gas inlet side as in the case of the honeycombfilter according to the embodiments of the present invention, since,upon supporting the catalyst, the area supporting the catalyst is nottoo narrow, the temperature difference between the area in which thecatalyst is supported on the catalyst supporting layer and the otherarea in which substantially no catalyst layer is formed does not becometoo large, and therefore a honeycomb filter having a high regenerationlimit value may be obtained more easily.

As described above, in the honeycomb filter according to the embodimentsof the present invention, the gas permeability coefficients of the cellwalls in the area in which the catalyst supporting layer is formed andthe area in which substantially no catalyst supporting layer is formedare controlled to be in a desirable range, and the catalyst supportinglayer is formed in a specific area. Therefore, the honeycomb filteraccording to the embodiments of the present invention is provided with ahigh regeneration limit value.

In the honeycomb filter according to the embodiments of the presentinvention, a catalyst is supported on the catalyst supporting layer.

In the honeycomb filter according to the embodiments of the presentinvention, a toxic component in exhaust gases can be purified and/orconverted by the catalyst supported on the catalyst supporting layer.

In the honeycomb filter according to the embodiments of the presentinvention, the thermal conductivity of the area in which substantiallyno catalyst supporting layer is formed is at least about 1.3 times andat most about 5.0 times larger than the thermal conductivity of the areain which the catalyst supporting layer is formed in the honeycombfilter.

In the honeycomb filter according to the embodiments of the presentinvention, since the area in which substantially no catalyst supportinglayer is formed in the honeycomb filter has a thermal conductivity atleast about 1.3 times and at most about 5.0 times larger than thethermal conductivity of the area in which the catalyst supporting layeris formed in the honeycomb filter, it may be easier to reduce occurrenceof a thermal impact caused by a temperature difference between the gasinlet side and the gas outlet side of the honeycomb filter. Therefore,the honeycomb filter according to the embodiments of the presentinvention is provided with e a higher regeneration limit value.

In the honeycomb filter according to the embodiments of the presentinvention, a main component of the honeycomb filter includes a carbideceramic, a nitride ceramic, or a complex of a metal and a carbideceramic, and a complex of a metal and a nitride ceramic.

Moreover, in the honeycomb filter according to the embodiments of thepresent invention, a main component of the honeycomb filter includessilicon carbide.

Since any of the above-mentioned materials for the main component of thehoneycomb filter has a high thermal conductivity, the honeycomb filtersaccording to the embodiments of the present invention are provided withan extremely high regeneration limit value.

First Embodiment

Referring to Figures, the following description will discuss a firstembodiment which is one of the embodiments of the present invention.

FIG. 2 is a perspective view that schematically shows one example of anembodiment of the honeycomb filter of the present invention. FIG. 3A isa perspective view that schematically shows one example of an embodimentof the honeycomb fired body forming the honeycomb filter of the presentinvention, and FIG. 3B is a cross-sectional view taken along the lineA-A in the FIG. 3A.

In a honeycomb filter 100, a plurality of honeycomb fired bodies asshown in FIGS. 3A and 3B are combined with one another by interposingsealing material layers (adhesive layers) 101 in between to configure aceramic block 103, and a sealing material layer (coat layer) 102 isformed on the outer periphery of the ceramic block 103.

The honeycomb fired body 110 includes porous silicon carbide as a maincomponent, and is formed by a large number of cells 111 which are placedin parallel with one another in a longitudinal direction (the directionshown by an arrow a in FIG. 3A) with a cell wall 113 therebetween, andthe cells 111 are each sealed with a plug 112 at either end thereof.Therefore, exhaust gases G having flown into the cell 111, having anopening on an end face on the gas outlet side, flow out of another cell111 after surely passing through the cell wall 113 which separates thecells 111.

Accordingly, the cell wall 113 functions as a filter for capturing PMand the like.

In the honeycomb filter of the present embodiment, gas permeabilitycoefficients k1 (μm²) and k2 (μm²) satisfy the following inequalities(1) and (2):(k1−k2)≦about 0.5  (1)about 1.0≦k1≦about 1.5  (2)

the gas permeability coefficient k1 being a gas permeability coefficientof a cell wall in the area in which substantially no catalyst supportinglayer is formed in the honeycomb filter, and the gas permeabilitycoefficient k2 being a gas permeability coefficient of a cell wall inthe area in which the catalyst supporting layer is formed in thehoneycomb filter.

Moreover, in the honeycomb filter 100, a catalyst supporting layer 10including an alumina having a platinum (Pt) catalyst supported thereonis formed in a predetermined area of the honeycomb filter 100. As aresult, the thermal conductivity of the area in which substantially nocatalyst supporting layer 10 is formed in the honeycomb filter 100becomes larger than the thermal conductivity of the area in which thecatalyst supporting layer 10 is formed in the honeycomb filter 100.

Furthermore, by supporting the catalyst on the catalyst supportinglayer, it becomes possible to accelerate conversion of toxic componentsin exhaust gases and/or burning of PM.

Referring to Figures, the following description will discuss thepredetermined area in which the catalyst supporting layer 10 is formed.

FIGS. 4A to 4D are cross-sectional views, each of which schematicallyshows one example of an embodiment of a honeycomb fired body in which acatalyst supporting layer is formed in a predetermined area.

More specifically, in the honeycomb fired body shown in FIG. 4A, acatalyst supporting layer 10 is formed in the area covering about 25% ofthe overall length L of the honeycomb fired body from the end face 21 onthe gas inlet side; in the honeycomb fired body shown in FIG. 4B, thecatalyst supporting layer 10 is formed in the area covering at leastabout 25% and at most about 50% of the overall length L of the honeycombfired body from the end face 21 on the gas inlet side; in the honeycombfired body shown in FIG. 4C, the catalyst supporting layer 10 is formedin the area covering about 50% of the overall length L of the honeycombfired body from the end face 21 on the gas inlet side; and in thehoneycomb fired body shown in FIG. 4D, the catalyst supporting layer 10is formed in the area covering about 90% of the overall length L of thehoneycomb fired body from the end face 21 on the gas inlet side.

Here, the overall length of the honeycomb filter is equal to the overalllength of the honeycomb fired body.

In each of the honeycomb fired bodies according to an embodiment of thepresent invention shown in FIGS. 4A to 4D, substantially no catalystsupporting layer is formed in the area covering about 10% of the overalllength L of the honeycomb fired body 110 from the end face 22 on the gasoutlet side (area B in FIGS. 4A to 4D, also referred to as anon-catalyst-supporting-layer area). Moreover, out of the area coveringabout 90% of the overall length L of the honeycomb fired body 110 fromthe end face 21 on the gas inlet side (area A in FIG. 4A), a catalystsupporting layer is formed in the area covering at least about 25% andat most about 90% (area C in FIGS. 4A to 4D, also referred to as acatalyst-supporting-layer area) of the overall length L of the honeycombfired body 110.

The area C in which the catalyst supporting layer 10 is formed may beprovided continuously from the end face 21 on the gas inlet side asshown in FIGS. 4A, 4C, and 4D, or alternatively, this area may also beprovided continuously from a position apart from the end face 21 on thegas inlet side as shown in FIG. 4B.

The catalyst supporting layer 10 may be formed on the surface of thecell wall 113, or alternatively, the catalyst supporting layer may alsobe formed inside the cell walls 113 in the honeycomb fired bodies.

Moreover, in the present embodiment, the honeycomb filter is designed soas to have the higher thermal conductivity in the area in whichsubstantially no catalyst supporting layer is formed in the honeycombfilter than the thermal conductivity in the area in which the catalystsupporting layer is formed in the honeycomb filter. More specifically,the thermal conductivity in the area in which substantially no catalystsupporting layer is formed in the honeycomb filter is at least about 1.3times and at most about 5.0 times larger than the thermal conductivityin the area in which the catalyst supporting layer is formed in thehoneycomb filter.

The thermal conductivities of the two areas are obtained by respectivelymeasuring thermal conductivities with respect to the cell walls at ameasuring portion 31 on the gas inlet side and a measuring portion 32 onthe gas outlet side, respectively shown in FIG. 3B.

Hereinafter, the following description will discuss the manufacturingmethods of a honeycomb filter of the present embodiment.

First, mixed powder is prepared as a ceramic material by dry-mixing asilicon carbide powder having a different average particle diameter, anorganic binder, and a pore-forming agent, and concurrently a liquidplasticizer, a lubricant, and water are mixed together to prepare amixed liquid. Then, the mixed powder and the mixed liquid are mixed byusing a wet mixing apparatus so that a wet mixture for manufacturing amolded body is prepared.

At this time, by changing the particle diameters of the ceramic materialand the pore-forming agent, as well as adjusting the blending ratios ofthe respective raw materials, the gas permeability coefficient of thecell wall in the area in which substantially no catalyst supportinglayer is formed can be controlled within a desirable range.

Subsequently, the wet mixture is charged into an extrusion-moldingmachine.

The wet mixture charged into the extrusion-molding machine isextrusion-molded so that a honeycomb molded body having a predeterminedshape is manufactured.

Next, the two ends of the honeycomb molded body are cut by using acutting machine so that the honeycomb molded body is cut into apredetermined length, and the resulting honeycomb molded body is driedby using a drying apparatus. Next, a predetermined amount of a plugmaterial paste to be formed into a plug is filled into ends on the gasoutlet side of a group of cells having openings on the end face on thegas inlet side, and ends on the gas inlet side of a group of cellshaving openings on the end face on the gas outlet side so that thecorresponding cells are sealed. Upon sealing the cells, a mask forsealing (plugging) the cells is applied on the end face of the honeycombmolded body (that is, the cut surface resulting from the cuttingprocess) so that the plug material paste is filled only into the cellsthat need to be sealed.

A honeycomb molded body with sealed cells is manufactured through theabove-mentioned processes.

Next, a degreasing process is carried out to heat organic matters of thehoneycomb molded body with the sealed cells in a degreasing furnace, andthen the resulting honeycomb degreased body is transported to a firingfurnace. Subsequently, a firing process is carried out to manufacture ahoneycomb fired body.

At this time, by adjusting firing conditions, the gas permeabilitycoefficient of the cell wall in the area in which substantially nocatalyst supporting layer is formed can be controlled within a desirablerange.

A sealing material paste is applied to a side face of the resultinghoneycomb fired body to form a sealing material layer (adhesive layer)thereon, and another honeycomb fired body is successively laminated withthe sealing material paste layer interposed therebetween. By repeatingthese processes, an aggregated body of a predetermined number ofhoneycomb fired bodies combined with one another is manufactured. Here,with respect to the sealing material paste, for example, a materialincluding an inorganic binder, an organic binder, and at least one ofinorganic fibers and inorganic particles may be used.

Next, the aggregated body of honeycomb fired bodies is heated so thatthe sealing material paste layers are dried and solidified to formsealing material layers (adhesive layers). Thereafter, a cutting processis carried out on the aggregated body of honeycomb fired bodies by usinga diamond cutter or the like to form a ceramic block, and the sealingmaterial paste is applied to a peripheral face of the ceramic block,then dried and solidified to form a sealing material layer (coat layer),thereby a honeycomb filter is manufactured.

Next, a catalyst supporting layer including alumina is formed in apredetermined area of the honeycomb filter, and a platinum catalyst issupported on the catalyst supporting layer. More specifically, thefollowing processes (a) and (b) are carried out.

(a) The honeycomb filter is immersed into an alumina solution containingalumina particles, with the face to be the end face on the gas inletside facing down, so that the predetermined area in which the catalystsupporting layer is to be formed is immersed in the alumina solution;thus, the alumina particles are selectively adhered to the predeterminedarea of the honeycomb filter.

Thereafter, the honeycomb filter is dried at a temperature of at leastabout 110° C. and at most about 200° C. for two hours, and the driedhoneycomb filter is heated and fired at a temperature of at least about500° C. and at most about 1000° C. so that the catalyst supporting layeris formed in the predetermined area of the honeycomb filter.

At this time, by adjusting the particle diameter of the aluminaparticles, the gas permeability coefficient of the cell wall in the areain which the catalyst supporting layer is formed can be controlledwithin a desirable range.

(b) Next, the honeycomb filter is immersed into a solution of a metalcompound containing platinum, with the face to be the end face on thegas inlet side facing down, so that the predetermined area in which thecatalyst supporting layer is formed is immersed in the solution, and theimmersed honeycomb filter is dried. Then, the dried honeycomb filter isheated and fired at a temperature of at least about 500 and at mostabout 800° C. under an inert atmosphere, so that a catalyst is supportedon the catalyst supporting layer.

Here, in the methods shown in the processes (a) and (b), the catalystsupporting layer is continuously formed from the end face on the gasinlet side of the honeycomb filter, and the catalyst is supported on thecatalyst supporting layer. However, in a case where, as shown in FIG.4B, the catalyst supporting layer is to be continuously formed from aposition apart from the end face on the gas inlet side of the honeycombfilter, and the catalyst is to be supported on the catalyst supportinglayer, for example, the following method may be used.

That is, prior to carrying out the process (a), an area on the gas inletside of the honeycomb filter in which substantially no catalystsupporting layer is to be formed is coated with a silicone resin, andthose processes up to the drying process of the process (a) are carriedout by using alumina particles with a platinum catalyst having beenpreliminarily applied. Then, the area is further heated to about 300° C.so that the silicone resin is fused and removed therefrom; successively,after the heating and firing processes of the process (a) are carriedout, the residual silicone resin on the honeycomb filter is dissolvedand removed therefrom by using an acid.

The following description will discuss effects produced by the honeycombfilter of the present embodiment.

(1) The gas permeability coefficients of the cell wall in the area (gasinlet side) in which the catalyst supporting layer is formed and thearea (gas outlet side) in which substantially no catalyst supportinglayer is formed are adjusted so that a difference of the gaspermeability coefficients of the cell walls is about 0.5 μm2 or less.

Since the difference of the gas permeability coefficients is controlledwithin this range, it may be easier to decrease the difference betweeneasiness of the exhaust gases passing through the cell wall on the gasinlet side and easiness of the exhaust gases passing through the cellwall on the gas outlet side.

Therefore, in exhaust gases having flowed into the cell, a largerproportion of the exhaust gases are more easily allowed to pass throughthe cell wall in the area in which the catalyst supporting layer isformed, so that the cell wall on the gas inlet side is more easilyallowed to capture more PM.

As a result, in the honeycomb filter of the present embodiment, theamount of PM to be captured on the cell wall on the gas outlet side ismore likely to be decreased, and upon burning PM, the temperaturedifference between the gas inlet side and the gas outlet side tends tobe decreased.

(2) Since the gas permeability coefficient k1 of the area in whichsubstantially no catalyst supporting layer is formed is about 1.5 μm2 orless, it may be easier to provide a honeycomb filter having a high PMcapturing efficiency.

(3) Since the gas permeability coefficient k1 of the area in whichsubstantially no catalyst supporting layer is formed is about 1.0 μm2 ormore, it may be easier to provide a honeycomb filter having a lowpressure loss.

(4) Since substantially no catalyst supporting layer is formed in thearea covering about 10% of the overall length of the honeycomb filterfrom the end face on the gas outlet side and the area is prepared as anarea made of a member having a high thermal conductivity, heat radiationfrom the end face neighborhood of the gas outlet side may be acceleratedmore easily. Consequently, since the temperature rise on the gas outletside of the honeycomb filter is more likely to be prevented, a thermalimpact caused by the temperature difference between the gas inlet sideand the gas outlet side of the honeycomb filter may be alleviated moreeasily.

(5) Since the catalyst supporting layer on which the catalyst issupported is formed in an area covering 25% or more of the overalllength of the honeycomb filter, the area on which the catalyst issupported tends to be sufficiently large. Therefore, in the regenerationprocess, it may be easier to prevent a large amount of heat from beinggenerated within a narrow area in the honeycomb filter.

(6) The synergistic effect obtained by controlling the gas permeabilitycoefficient of the area in which substantially no catalyst supportinglayer is formed within a desirable range and forming the catalystsupporting layer in a desirable area improve the regeneration limitvalue.

Consequently, it may be easier to provide a honeycomb filter having ahigh regeneration limit value.

EXAMPLES

The following description will discuss examples which more specificallydisclose the first embodiment of the present invention; however, thepresent invention is not limited to those examples.

In the following Examples, Reference Examples, and Comparative Examples,honeycomb filters were manufactured to have the gas permeabilitycoefficients k1 (a cell wall in the area in which substantially nocatalyst supporting layer is formed) and k2 (a cell wall in the area inwhich a catalyst supporting layer is formed) of different values, andhoneycomb filters were also manufactured to have the various formationrange of a catalyst supporting layer; thus, measurements of respectivecharacteristics were carried out thereon.

Here, the honeycomb filter prior to the formation of the catalystsupporting layer, manufactured in each of the Examples and the like, isreferred to as “base member”.

Example 1 Manufacturing of Honeycomb Fired Body

An amount of 52.8% by weight of coarse powder of silicon carbide havingan average particle diameter of 22 μm and 22.6% by weight of fine powderof silicon carbide having an average particle diameter of 0.5 μm weremixed. To the resulting mixture, 2.1% by weight of acrylic resin havingan average particle diameter of 20 μm, 4.6% by weight of an organicbinder (methylcellulose), 2.8% by weight of a lubricant (UNILUB, made byNOF Corporation), 1.3% by weight of glycerin, and 13.8% by weight ofwater were added, and then kneaded to prepare a wet-mixture composite.Then, the wet-mixture composite was extrusion molded so that a rawhoneycomb molded body having virtually the same shape as the shape shownin FIG. 3A, with no cells being sealed, was manufactured.

Next, the raw honeycomb molded body was dried by using a microwavedrying apparatus to obtain a dried body of the honeycomb molded body.Thereafter, a paste having the same composition as the raw molded bodywas filled into predetermined cells, and then again dried by a dryingapparatus.

The dried honeycomb molded body was degreased at 400° C., and then firedat 2150° C. under normal pressure argon atmosphere for 3 hours so as tomanufacture a honeycomb fired body formed by a silicon carbide sinteredbody with a porosity of 45%, an average pore diameter of 13.0 μm, a sizeof 34.3 mm×34.3 mm×150 mm, the number of cells (cell density) of300/inch2 (46.5 pcs/cm2) and a thickness of the cell wall of 0.25 mm (10mil).

(Manufacturing of Honeycomb Filter)

A large number of honeycomb fired bodies were bonded to one another byusing a heat resistant sealing material paste containing 30% by weightof alumina fibers having an average fiber length of 20 μm, 21% by weightof silicon carbide particles having an average particle diameter of 0.6μm, 15% by weight of silica sol, 5.6% by weight of carboxymethylcellulose, and 28.4% by weight of water. The bonded honeycomb firedbodies were dried at 120° C., and then cut by using a diamond cutter sothat a round pillar-shaped ceramic block having the sealing materiallayer (adhesive layer) with a thickness of 1.0 mm was manufactured.

Next, a sealing material paste layer having a thickness of 0.2 mm wasformed on the peripheral portion of the ceramic block by using thesealing material paste. Further, the sealing material paste layer wasdried at 120° C. so that a round pillar-shaped honeycomb filter having asize of 143.8 mm in diameter×150 mm in length, with a sealing materiallayer (coat layer) formed on the periphery thereof, was manufactured.The average particle diameter of the raw material, the composition ofthe raw material, and the firing temperature of the manufacturedhoneycomb filter (honeycomb fired body) are shown in Table 1. Moreover,characteristics of the manufactured honeycomb filter (honeycomb firedbody) are shown in Table 2.

The honeycomb filter manufactured in the present Example corresponds tothe base member 3 among the base members 1 to 8 shown in Table 1 and 2.

Here, in Table 2, the section of “Cell Structure” indicates thethickness (mil) of the cell wall and cell density (pcs/inch²).

TABLE 1 Raw material average particle diameter(μm) Raw materialcomponent (% by weight) Firing SiC coarse SiC fine Pore forming SiCcoarse SiC fine Methyl Acrylic UNILU temperature powder powder agentpowder powder cellulose resin B Glycerin Water (° C.) Base 11 0.5 0 54.623.4 4.3 0 2.6 1.2 13.9 2250 member 1 Base 11 0.5 0 54.6 23.4 4.3 0 2.61.2 13.9 2200 member 2 Base 22 0.5 20 52.8 22.6 4.6 2.1 2.8 1.3 13.92150 member 3 Base 22 0.5 20 52.2 22.4 4.8 2.6 2.9 1.3 13.8 2200 member4 Base 25 0.5 20 52.2 22.4 4.8 2.6 2.9 1.3 13.8 2200 member 5 Base 110.5 0 54.6 23.4 4.3 0 2.6 1.2 13.9 2200 member 6 Base 11 0.5 20 51.021.9 4.9 3.1 2.9 1.5 14.7 2200 member 7 Base 30 0.5 20 43.5 18.6 6.0 8.53.6 1.6 18.2 2200 member 8

TABLE 2 Honeycomb filter characteristics Average Gas pore permeabilitydiameter Porosity coefficient Cell (μm) (%) k1 (μm²) structure* Basemember 1 11 42 0.9 10/300 Base member 2 12 42 1.0 10/300 Base member 313 45 1.2 10/300 Base member 4 15 48 1.5 10/300 Base member 5 18 48 1.610/300 Base member 6 10 42 0.7 14/200 Base member 7 10 50 0.9 12/300Base member 8 20 65 2.6 14/200 *Cell structure is indicated by thickness(mil) of a cell wall/cell density (pcs/inch²). *Gas permeabilitycoefficient k1 indicates permeability coefficient on the gas outletside.(Forming of Catalyst Supporting Layer)

γ-alumina particles having an average particle diameter of 0.9 μm weremixed with a sufficient amount of water, and stirred to prepare analumina slurry. A honeycomb filter was immersed in this alumina slurryup to an area covering 33.3% of its overall length (area covering 50 mmfrom end face on the gas inlet side), with its end face on the gas inletside facing down, and maintained in this state for one minute.

Next, this honeycomb filter was heated at 110° C. for one hour to bedried, and further fired at 700° C. for one hour so that a catalystsupporting layer was formed in the area covering 33.3% of its overalllength from the end face on the gas inlet side of the honeycomb filter.

At this time, the immersing process into the alumina slurry, dryingprocess, and firing process were repeatedly carried out so that theformation amount of the catalyst supporting layer became 60 g per 1liter volume of the area in which the catalyst supporting layer isformed in the honeycomb filter.

(Supporting of Pt Catalyst)

The honeycomb filter was immersed in diammine dinitro platinum nitricacid ([Pt(NH₃)₂(NO₂)₂]HNO₃, platinum concentration of 4.53% by weight)up to an area covering 33.3% of its overall length, with its end face onthe inlet side of the honeycomb filter facing down and maintained inthis state for one minute.

Next, the honeycomb filter was dried at 110° C. for two hours andfurther fired at 500° C. for one hour under nitrogen atmosphere so thata platinum catalyst was supported on the catalyst supporting layer.

The amount of the supported catalyst was 3 g of platinum with respect to20 g of alumina of the catalyst supporting layer.

By carrying out the aforementioned processes, a honeycomb filter inwhich a catalyst supporting layer including alumina was formed in apredetermined area, and a platinum catalyst was supported on thecatalyst supporting layer, was manufactured.

The honeycomb filter manufactured as described above was measured asfollows.

(Measurement of Gas Permeability Coefficient)

A honeycomb filter was cut in a direction perpendicular to thelongitudinal direction of the honeycomb filter at an interface betweenan area in which a catalyst supporting layer was formed and an area inwhich substantially no catalyst supporting layer was formed.

Moreover, out of the cells exposed to the cut face, a paste having thesame composition as that of the raw molded body was filled into thecells each having the other end that was not sealed, and the paste wasdried by using a drying apparatus.

Thus, a honeycomb filter for use in measuring a pressure loss on the gasinlet side, having only the area in which the catalyst supporting layeris formed and having the cells each sealed at either one of the ends,and a honeycomb filter for use in measuring a pressure loss on the gasoutlet side, having only the area in which substantially no catalystsupporting layer is formed and having the cells each sealed at eitherone of the ends were manufactured.

The pressure loss of each of the honeycomb filter for use in measuring apressure loss on the gas inlet side and the honeycomb filter for use inmeasuring a pressure loss on the gas outlet side was measured by apressure loss measuring apparatus 210 as shown in FIG. 5.

In this pressure loss measuring apparatus 210, a honeycomb filter 100fixed in a metal casing 213 was placed in an exhaust gas pipe 212 of ablower 211, and a pressure gauge 214 was installed so as to detectpressures at the front and back of the honeycomb filter 100.

Here, the pressure loss measuring apparatus 210 was driven with the flowrate of exhaust gases from the blower 211 being made constant, and after5 minutes from the start of the driving, a pressure difference (pressureloss) was measured.

While the gas flow rate of exhaust gases from the blower 211 was beingchanged, the pressure loss ΔP relative to each gas flow rate Q wasmeasured. Then, as shown in FIG. 1, a graph in which Q is plotted on thehorizontal axis and ΔP/Q being plotted on the longitudinal axis wasdrawn, and the value of an intercept was calculated.

Based upon the value of the intercept, gas permeability coefficients k1(a cell wall in the area in which substantially no catalyst supportinglayer is formed) and k2 (a cell wall in the area in which a catalystsupporting layer is formed) were obtained.

(Measurement of Thermal Conductivity)

As shown in FIG. 3B, portions of cell walls were cut from the honeycombfilter to form a measuring portion 31 on the gas inlet side and ameasuring portion 32 on the gas outlet side, and the thermalconductivity of each of cell walls in the respective measuring portionswas measured by a laser flash method.

(Measurement of Regeneration Limit Value)

As shown in FIG. 6, a honeycomb filter is placed in an exhaust passageof an engine so that an exhaust gas purifying apparatus was formed, andthe regeneration limit value was measured.

An exhaust gas purifying apparatus 220 was mainly configured with ahoneycomb filter 100, a casing 221 that covers the outside of thehoneycomb filter 100, and a holding sealing material 222 interposedbetween the honeycomb filter 100 and the casing 221. Further, anintroducing pipe 224, which was coupled to an internal combustion enginesuch as an engine, was connected to the end portion of the casing 221 onthe side from which exhaust gases were introduced, and an exhaust pipe225 coupled to the outside was connected to the other end portion of thecasing 221. Here, in FIG. 6, arrows show flows of exhaust gases.

The engine was driven at the number of revolutions of 3000 min-1 and atorque of 50 Nm for a predetermined period of time so that apredetermined amount of PM was captured. Thereafter, the engine wasdriven in full load at the number of revolutions of 4000 min-1, and atthe time when the filter temperature became constant at about 700° C.,the engine was driven slowly at the number of revolutions of 1050 min-1and a torque of 30 Nm so that PM was forcefully burned.

Then, this experiment was carried out in which a regeneration processwas executed while the amount of captured PM was being changed, so thatwhether or not any cracks occurred in the filter was examined. Here, themaximum amount of PM which would not cause any cracks was defined as theregeneration limit value.

(Measurement of PM Thickness Difference)

The exhaust gas purifying apparatus 220 used for measuring theregeneration limit value was driven to capture 10 g/L of PM in ahoneycomb filter. Thereafter, the honeycomb filter was taken out of theexhaust gas purifying apparatus 220, and the honeycomb filter was cutalong a direction perpendicular to the longitudinal direction thereof ata position of 25 mm and at a position of 100 mm in the longitudinaldirection of the honeycomb filter from the end face on the gas inletside.

These two cut positions respectively corresponded to center of the areain which the catalyst supporting layer was formed and center of the areain which substantially no catalyst supporting layer was formed, to thelongitudinal direction of the honeycomb filter.

FIG. 7 is a magnified cross-sectional view that schematically shows amethod for measuring a thickness of PM on the cut plane of the honeycombfilter.

FIG. 7 schematically shows a magnified view of one portion of the cutplane of the honeycomb filter observed by an electronic microscope(SEM), and PM 115 is captured on the cell wall 113 forming the cell 111shown in the center of FIG. 7.

At this time, the thickness of the captured PM 115 was defined as adistance from the surface of the cell wall 113 to the surface of thecaptured PM 115, that is, a thickness indicated by t in FIG. 7.

The thickness of PM, defined as described above, was measured at acut-out position on the gas inlet side and at a cut-out position on thegas outlet side of the honeycomb filter.

As a result, in the honeycomb filter of the present embodiment, thethickness of PM captured on the cell wall on the gas outlet side becamethicker than the thickness of PM captured on the cell wall on the gasinlet side. Here, the difference between the thickness of PM captured onthe cell wall on the gas outlet side and the thickness of PM captured onthe cell wall on the gas inlet side was obtained as “PM thicknessdifference”.

(Measurement of Capturing Efficiency)

By using a capturing efficiency measuring apparatus 230 as shown in FIG.8, the capturing efficiency of PM was measured. FIG. 8 is an explanatoryview that shows the capturing efficiency measuring device.

This capturing efficiency measuring apparatus 230 is configured as ascanning mobility particle sizer (SMPS) that is provided with 2 litersof a common-rail-type diesel engine 231, an exhaust gas pipe 232 thatallows the exhaust gases from the engine 231 to pass through, a metalcasing 233 that is connected to the exhaust gas pipe 232, and securesthe honeycomb filter 100 wrapped with an alumina mat, a sampler 235 usedfor sampling the exhaust gases before passing through the honeycombfilter 100, a sampler 236 used for sampling the exhaust gases afterpassing through the honeycomb filter 100, a diluter 237 that dilutes theexhaust gases sampled by the samplers 235 and 236, and a PM counter 238(a condensation particle counter 3022A-S, made by TSI, Inc.) thatmeasures the amount of PM contained in the diluted the exhaust gases.

Next, the measuring procedures are described. The engine 231 was drivenat the number of revolutions of 2000 min⁻¹ and a torque of 47 Nm, andexhaust gases from the engine 231 were allowed to flow through thehoneycomb filter 100. At this time, the amount of PM P₀ in the exhaustgases before passing through the honeycomb filter 100 and the amount ofPM P₁ in the exhaust gases after passing through the honeycomb filter100 were obtained by using the PM counter 238. Then, the capturingefficiency was calculated based upon the following equation.Capturing efficiency (%)=(P ₀ −P ₁)×100/P ₀(Measurement of Pressure Loss of Honeycomb Filter)

By using the above-mentioned pressure loss measuring apparatus 210, thepressure loss of the honeycomb filter was measured in the same manner asthe method used upon measuring the gas permeability coefficient.

Here, the blower 211 was driven so that the flow rate of exhaust gaseswas 750 m³/h, and after 5 minutes from the start of the driving, apressure difference (pressure loss) was measured.

Table 3 collectively shows: the base member of the honeycomb filter; theformation range, the formation position, and the formation amount of thecatalyst supporting layer; and the particle size of alumina particlesused for forming the catalyst supporting layer of the honeycomb filtermanufactured in Example 1. Further, Table 4 collectively shows themeasurement results of the gas permeability coefficient, the thermalconductivity, the PM thickness difference, the regeneration limit value,the pressure loss, and the capturing efficiency.

Here, the formation position of the catalyst supporting layer isindicated by a distance (mm) from the gas inlet side, supposing that theposition of the end face on the gas inlet side is 0 mm and that theposition of the end face on the gas outlet side is 150.0 mm. In Example1, since the catalyst supporting layer is formed in an area covering50.0 mm from the end face on the gas inlet side, the position is givenas “0-50.0”.

Moreover, the formation amount of the catalyst supporting layer is givenas a formation amount per 1 liter volume of the area of the honeycombfilter in which the catalyst supporting layer is formed.

Example 2 Comparative Example 1

The same base members 3 as that of Example 1 were manufactured, andhoneycomb filters were manufactured in the same manner as in Example 1,except that the average particle diameter of γ-alumina particles usedupon forming a catalyst supporting layer was changed as shown in Table 3to form a catalyst supporting layer.

The respective characteristics of each of these honeycomb filters weremeasured in the same manner as in Example 1, and the results of theseare collectively shown in Table 4.

TABLE 3 Catalyst supporting layer on inlet side Alumina FormationFormation Formation particle Base range position amount diameter member(%) (mm) (g/L) (μm) Example 1 3 33.3 0-50.0 60.0 0.9 Example 2 3 33.30-50.0 60.0 1.5 Comparative 3 33.3 0-50.0 60.0 2.6 Example 1

TABLE 4 Gas permeability coefficient (μm²) Thermal conductivity (W/mK)PM thickness Regeneration Pressure Capturing Inlet Outlet Inlet Outletdifference limit loss efficiency side k2 side k1 k1 − k2 side side ratio(μm) value (g/L) (kPa) (%) Example 1 0.9 1.2 0.3 8.2 17.1 2.09 27 6.68.0 86 Example 2 0.7 1.2 0.5 8.4 17.1 2.04 28 6.8 7.9 89 Comparative 0.61.2 0.6 8.7 17.1 1.97 52 3.8 8.3 82 Example 1

Tables 3 and 4 show characteristics of honeycomb filters in a case wherethe gas permeability k2 on the gas inlet side is changed, and in Tables3 and 4, Example 1 on the uppermost column to Comparative Example 1 onthe lowermost column are shown in the descending order of the gaspermeability coefficient k2 on the gas inlet side.

Here, the gas permeability coefficient k1 on the gas outlet side wasconstant at 1.2 μm2, and was always larger than the gas permeabilitycoefficient k2; therefore, the value of (k1−k2) became smaller as thegas permeability coefficient k2 became larger.

Based upon the results shown in Table 4, FIG. 9 is a graph plotted toshow the relationship among the difference in the gas permeabilitycoefficients on the gas inlet side and the gas outlet side, theregeneration limit value, and the PM thickness difference.

Table 4 and FIG. 9 show that in a case where the difference in the gaspermeability coefficients on the gas inlet side and the gas outlet sidewas 0.5 μm2 or less, the regeneration limit value was as high as 6.6 g/Lor more.

Moreover, in a case where the difference in the gas permeabilitycoefficients on the gas inlet side and the gas outlet side was 0.5 m² orless, the PM thickness difference was decreased.

From these results, by setting the difference in the gas permeabilitycoefficients on the gas inlet side and the gas outlet side to about 0.5μm2 or less, it may be easier to comparatively reduce the amount of PMto be captured on the cell wall on the gas outlet side, and consequentlyto provide a honeycomb filter having a high regeneration limit value.

Examples 3 and 4 Comparative Examples 2 and 3

By changing the average particle diameter of coarse powder of siliconcarbide in the mixture composite, the composition of the raw material,and the firing temperature as shown in Table 1, base members 1, 2, 4,and 5 having characteristics as shown in Table 2 were manufactured.

Each of these base members 1 to 5 had different gas permeabilitycoefficient k1 on the gas outlet side ranging from 0.9 to 1.6 μm2.

On each of these base members, a catalyst supporting layer was formed byusing γ-alumina particles having an average particle diameter of 1.5 μmin the same manner as in Example 2 to manufacture a honeycomb filter asshown in Table 5.

The respective characteristics of each of these honeycomb filters weremeasured in the same manner as in Example 1, and the results thereof areshown in Table 6 together with the results of Example 2.

TABLE 5 Catalyst supporting layer on inlet side Alumina FormationFormation Formation particle Base range position amount diameter member(%) (mm) (g/L) (μm) Comparative 1 33.3 0-50.0 60.0 1.5 Example 2 Example3 2 33.3 0-50.0 60.0 1.5 Example 2 3 33.3 0-50.0 60.0 1.5 Example 4 433.3 0-50.0 60.0 1.5 Comparative 5 33.3 0-50.0 60.0 1.5 Example 3

TABLE 6 Gas permeability coefficient (μm²) Thermal conductivity (W/mK)PM thickness Regeneration Pressure Capturing Inlet Outlet Inlet Outletdifference limit loss efficiency side k2 side k1 k1 − k2 side side ratio(μm) value (g/L) (kPa) (%) Comparative 0.4 0.9 0.5 8.7 18.7 2.15 42 5.810.9 92 Example 2 Example 3 0.5 1.0 0.5 8.7 18.5 2.13 32 7.4 8.8 93Example 2 0.7 1.2 0.5 8.4 17.1 2.04 28 6.8 7.9 89 Example 4 1.1 1.5 0.48.1 16.6 2.05 34 6.2 7.7 83 Comparative 1.3 1.6 0.3 8.1 16.4 2.02 38 5.17.7 68 Example 3

Tables 5 and 6 show characteristics of a honeycomb filter in a casewhere the gas permeability coefficient k1 on the gas outlet side ischanged, and in Tables 5 and 6, Comparative Example 2 on the uppermostcolumn to Comparative Example 3 on the lowermost column is shown in theascending order of the gas permeability coefficient k1 on the gas outletside.

At this time, in Examples 3, 2, and 4, the gas permeability coefficientsk1 on the gas outlet side were in a range from 1.0 to 1.5 μm2;therefore, it became possible to provide honeycomb filters with a lowpressure loss and a high capturing efficiency.

On the other hand, it is presumed that in Comparative Example 2, the gaspermeability coefficient k1 on the gas outlet side was as small as 0.9μm², resulting in a high pressure loss of 10.9 kPa, while in ComparativeExample 3, the gas permeability coefficient k1 on the gas outlet sidewas as high as 1.6 μm², resulting in a low capturing efficiency of 68%.

Examples 5 to 7 Comparative Examples 4 and 5

Honeycomb filters (base members 3) were manufactured in the same manneras in Example 1, and a catalyst supporting layer was formed on each ofthese base members by using γ-alumina particles having an averageparticle diameter of 0.9 μm in the same manner as in Example 1. At thistime, by changing the depth to which the honeycomb filters were immersedinto a slurry, the catalyst supporting layers were formed in a rangefrom 20 to 100% of the overall length of the honeycomb filter as shownin Table 7.

Moreover, the number of repeated cycles of processes of immersing intoan alumina slurry, drying, and firing was changed so that the formationamount of each catalyst supporting layer per 1 liter volume in the areain which the catalyst supporting layer was formed in the honeycombfilter was set to each of values shown in Table 7. This formation amountwas designed so that the formation amount of the catalyst supportinglayer became 20 g per 1 liter volume of the entire honeycomb filter.

The catalyst was supported on the same area in which the catalystsupporting layer was formed.

The respective characteristics of these honeycomb filters were measuredin the same manner as in Example 1, and the results thereof arecollectively shown in Table 8 together with the results of Example 1.

TABLE 7 Catalyst supporting layer on inlet side Alumina FormationFormation Formation particle Base range position amount diameter member(%) (mm) (g/L) (μm) Comparative 3 20.0 0-30.0 100.0 0.9 Example 4Example 5 3 25.0 0-37.5 80.0 0.9 Example 1 3 33.3 0-50.0 60.0 0.9Example 6 3 50.0 0-75.0 40.0 0.9 Example 7 3 90.0  0-135.0 22.2 0.9Comparative 3 100.0  0-150.0 20.0 0.9 Example 5

TABLE 8 Gas permeability coefficient (μm²) Thermal conductivity (W/mK)PM thickness Regeneration Pressure Capturing Inlet Outlet Inlet Outletdifference limit loss efficiency side k2 side k1 k1 − k2 side side ratio(μm) value (g/L) (kPa) (%) Comparative 0.7 1.2 0.5 5.9 17.1 2.90 59 3.78.4 83 Example 4 Example 5 0.8 1.2 0.4 7.0 17.1 2.44 39 6.3 8.3 85Example 1 0.9 1.2 0.3 8.2 17.1 2.09 27 6.8 8.0 86 Example 6 1.0 1.2 0.29.9 17.1 1.73 25 6.8 8.0 88 Example 7 1.1 1.2 0.1 12.5 17.1 1.37 25 6.67.9 88 Comparative 1.1 NA 12.8 NA 11 3.3 7.9 86 Example 5

Tables 7 and 8 show characteristics of honeycomb filters in a case wherea range for forming the catalyst supporting layer is changed, and inTable 7 and 8, Comparative Example 4 on the uppermost column toComparative Example 5 on the lowermost column are shown in the ascendingorder of the formation ranges of the catalyst supporting layer.

Based upon the results indicated by Table 8, FIG. 10 shows a graphplotted to show the relationship between the formation range of thecatalyst supporting layer and the regeneration limit value.

As indicated by Table 8 and FIG. 10, when the formation ranges of thecatalyst supporting layer on the gas inlet side ranging from 25 to 90%,the regeneration limit value was as high as 6.3 g/L or more, and incases where the formation ranges of the catalyst supporting layer on thegas inlet side were 20% and 100%, the respective regeneration limitvalues were as low as 3.7 g/L and 3.3 g/L.

In other words, by forming the catalyst supporting layer in an areawithin the range specified by the embodiment of the present invention,it became possible to provide a honeycomb filter having a highregeneration limit value.

Reference Example 1

A honeycomb filter was manufactured in the same manner as in Example 5,except that the average particle diameter of γ-alumina particles usedupon forming the catalyst supporting layer was changed to 0.5 μm shownin Table 9.

The respective characteristics of the honeycomb filter were measured inthe same manner as in Example 5, and the results thereof arecollectively shown in Table 10 together with the results of Example 5.

TABLE 9 Catalyst supporting layer on inlet side Alumina FormationFormation Formation particle Base range position amount diameter member(%) (mm) (g/L) (μm) Example 5 3 25.0 0-37.5 80.0 0.9 Reference 3 25.00-37.5 80.0 0.5 Example 1

TABLE 10 Gas permeability coefficient (μm²) Thermal conductivity (W/mK)PM thickness Regeneration Pressure Capturing Inlet Outlet Inlet Outletdifference limit loss efficiency side k2 side k1 k1 − k2 side side ratio(μm) value (g/L) (kPa) (%) Example 5 0.8 1.2 0.4 7.0 17.1 2.44 39 6.38.3 85 Reference 0.9 1.2 0.3 3.4 17.1 5.03 35 5.9 8.1 82 Example 1

In the honeycomb filter manufactured in Reference Example 1, the ratioof the thermal conductivity of the gas outlet side measuring portion tothe thermal conductivity of the gas inlet side measuring portion is ashigh as 5.03, and the regeneration limit value was as comparatively lowas 5.9 g/L.

Comparative Examples 6, 7, and 8

By changing the average particle diameter of coarse powder of siliconcarbide in the mixture composite, the composition of the raw material,and the firing temperature to values as shown in Table 1, the averagepore diameter, the porosity, and the gas permeability coefficient k1 onthe gas outlet side were controlled. Further, by changing the design ofthe die used upon extrusion-molding, the cell structure was controlled.Based upon these, base members 6 to 8 having characteristics shown inTable 2 were manufactured. On each of these base members 6 to 8, thecatalyst supporting layer made of γ-alumina was formed within a range asshown in Table 11 by using a sol-gel method and a catalyst was supportedon the catalyst supporting layer to manufacture a honeycomb filter.

The respective characteristics of each of these honeycomb filters weremeasured in the same manner as in Example 1, and the results thereof arecollectively shown in Table 12.

TABLE 11 Catalyst supporting layer on inlet side Alumina FormationFormation Formation particle Base range position amount diameter member(%) (mm) (g/L) (μm) Comparative 6 33.3 0-50.0 60.0 NA Example 6Comparative 7 33.3 0-50.0 60.0 NA Example 7 Comparative 8 33.3 0-50.060.0 NA Example 8

TABLE 12 Gas permeability coefficient (μm²) Thermal conductivity (W/mK)PM thickness Regeneration Pressure Capturing Inlet Outlet Inlet Outletdifference limit loss efficiency side k2 side k1 k1 − k2 side side ratio(μm) value (g/L) (kPa) (%) Comparative 0.3 0.7 0.4 9.3 18.2 1.96 70 3.011.2 85 Example 6 Comparative 0.4 0.9 0.5 9.7 15.5 1.60 76 2.8 9.4 76Example 7 Comparative 1.4 2.6 1.2 5.2 7.8 1.50 40 2.5 7.5 65 Example 8

These honeycomb filters are conventionally known honeycomb filters.However, with regard to the honeycomb filters according to ComparativeExamples 6 and 7, the gas permeability coefficient k1 on the gas outletside was small and the pressure loss was large. Moreover, it is presumedthat with regard to the honeycomb filter of Comparative Example 8, thedifference in gas permeability coefficients (k1−k2) was large so thatthe regeneration limit value became low, and since the gas permeabilitycoefficient k2 on the gas inlet side was large, the capturing efficiencybecame low.

Second Embodiment

In the first embodiment, the honeycomb filter has a structure in which aplurality of honeycomb fired bodies are combined with one another byinterposing sealing material layers (adhesive layers); however, thehoneycomb filter according to the present embodiment is a honeycombfilter formed by a single honeycomb fired body.

In the present specification, the former honeycomb filter is referred toas an aggregated honeycomb filter and the latter honeycomb filter isreferred to as an integral honeycomb filter.

When manufacturing such an integral honeycomb filter, a honeycomb moldedbody is manufactured through the same method as the method formanufacturing an aggregated honeycomb filter, except that the size of ahoneycomb molded body to be molded through the extrusion-molding islarger than the size of a honeycomb molded body forming the aggregatedhoneycomb filter. Thereafter, the integral honeycomb filter can bemanufactured through the same method as that for manufacturing theaggregated honeycomb filter of the first embodiment.

As a main constituent material for the integral honeycomb filter,cordierite and aluminum titanate are desirably used due to theirsuperior thermal impact resistance, and also in the present embodiment,it is possible to obtain the same effects (1) to (6) of the firstembodiment.

Other Embodiments

With respect to the shape of the honeycomb filter according to theembodiments of the present invention, it is not particularly limited tothe round pillar shape shown in FIG. 1, and the honeycomb filter mayhave any desired pillar shape, such as a cylindroid shape and arectangular pillar shape.

The porosity of the honeycomb filter according to the embodiments of thepresent invention is desirably at least about 30% and at most about 70%.

This structure may make it easier to maintain sufficient strength in thehoneycomb filter and to maintain a low level resistance at the time ofpassage of exhaust gases through the cell wall.

In contrast, the porosity of less than 30% tends to cause clogging inthe cell wall in an early stage, while the porosity of more than 70%tends to cause a decrease in strength of the honeycomb filter with theresult that the honeycomb filter might be easily broken.

Here, the porosity can be measured through conventionally known methodssuch as a mercury injection method, Archimedes method, and a measuringmethod using a scanning electronic microscope (SEM).

The cell density on a cross section perpendicular to the longitudinaldirection of the honeycomb filter is not particularly limited. However,a desirable lower limit is about 31.0 pcs/cm2 (about 200 pcs/in2) and adesirable upper limit is about 93.0 pcs/cm2 (about 600 pcs/in2). A moredesirable lower limit is about 38.8 pcs/cm2 (about 250 pcs/in2) and amore desirable upper limit is about 77.5 pcs/cm2 (about 500 pcs/in2).

The main component of constituent materials of the honeycomb filter isnot limited to silicon carbide, and may include other ceramic materials:a nitride ceramic such as aluminum nitride, silicon nitride, boronnitride, and titanium nitride; a carbide ceramic such as zirconiumcarbide, titanium carbide, tantalum carbide, and tungsten carbide; acomplex of a metal and a nitride ceramic; a complex of a metal and acarbide ceramic; and the like.

Moreover, the constituent materials also include a silicon-containingceramic formed by compounding a metal silicon into the above-mentionedceramics and a ceramic material such as a ceramic bonded by a silicon ora silicate compound.

In the case of the aggregated honeycomb filter as described in the firstembodiment, silicon carbide is particularly desirably used as the maincomponent of the constituent materials of the honeycomb filter.

This is because silicon carbide is excellent in heat resistant property,mechanical strength, thermal conductivity, and the like.

Moreover, a material formed by compounding metal silicon with siliconcarbide (silicon-containing silicon carbide) is also desirable.

Although the particle diameter of silicon carbide powder used in the wetmixture is not particularly limited as long as the gas permeabilitycoefficient k1 on the gas outlet side can be controlled within adesirable range, it is desirable to use the silicon carbide powder thattends not to cause the case where the size of the honeycomb fired bodymanufactured by the following firing treatment becomes smaller than thatof the honeycomb molded body. For example, it is desirable to use acombination of 100 parts by weight of the powder having an averageparticle diameter of at least about 1.0 μm and at most about 50.0 μm andat least about 5 parts by weight and at most about 65 parts by weight ofthe powder having an average particle diameter of at least about 0.1 μmand at most about 1.0 μm.

The organic binder in the wet mixture is not particularly limited, andexamples thereof include carboxymethyl cellulose, hydroxyethylcellulose, polyethylene glycol, and the like. Out of these,methylcellulose is more desirably used. In general, the blending amountof the organic binder is desirably at least about 1 part by weight andat most about 10 parts by weight with respect to 100 parts by weight ofthe ceramic powder.

A plasticizer and a lubricant to be used upon preparing the wet mixtureare not particularly limited, and for example, glycerin or the like maybe used as the plasticizer. Moreover, as the lubricant, for example,polyoxy alkylene-based compounds, such as polyoxyethylene alkyl etherand polyoxypropylene alkyl ether, may be used.

Specific examples of the lubricant include polyoxyethylene monobutylether, polyoxypropylene monobutyl ether, and the like.

Here, the plasticizer and the lubricant are not necessarily contained inthe wet mixture depending on cases.

Upon preparing the wet mixture, a dispersant solution may be used, andexamples of the dispersant solution include water, an organic solventsuch as benzene, and alcohol such as methanol, and the like.

Moreover, a molding auxiliary may be added to the wet mixture.

The molding auxiliary is not particularly limited, and examples thereofinclude ethylene glycol, dextrin, fatty acid, fatty acid soap,polyalcohol, and the like.

Furthermore, a pore-forming agent, such as balloons that are fine hollowspheres including an oxide-based ceramic, spherical acrylic particles,and graphite may be added to the wet mixture, if necessary.

The gas permeability coefficient of the honeycomb filter can becontrolled by adjusting the particle diameter of the pore forming agentto be added.

With respect to the balloons, not particularly limited, for example,alumina balloons, glass micro-balloons, shirasu balloons, fly ashballoons (FA balloons), mullite balloons, and the like may be used. Outof these, alumina balloons are more desirably used.

Moreover, the content of organic components in the wet mixture isdesirably about 10% by weight or less, and the content of moisture isdesirably at least about 8% by weight and at most about 30% by weight.

Although a plug material paste used for sealing cells is notparticularly limited, the plug material paste that allows the plugsmanufactured through post processes to have a porosity of at least about30% and at most about 75% is desirably used. For example, the samematerial as that of the wet mixture may be used.

Examples of the inorganic binder in the sealing material paste includesilica sol, alumina sol, and the like. Each of these may be used aloneor two or more kinds of these may be used in combination. Silica sol ismore desirably used among the inorganic binders.

Examples of the organic binder in the sealing material paste includepolyvinyl alcohol, methyl cellulose, ethyl cellulose, carboxymethylcellulose, and the like. Each of these may be used alone or two or morekinds of these may be used in combination. Carboxymethyl cellulose ismore desirably used among the organic binders.

Examples of the inorganic fibers in the sealing material paste includeceramic fibers and the like made from silica-alumina, mullite, alumina,silica, or the like. Each of these may be used alone or two or morekinds of these may be used in combination. Alumina fibers are moredesirably used among the inorganic fibers.

Examples of the inorganic particles in the sealing material pasteinclude carbides, nitrides, and the like, and specific examples thereofinclude inorganic powder and the like made from silicon carbide, siliconnitride, boron nitride, and the like. Each of these may be used alone,or two or more kinds of these may be used in combination. Out of theinorganic particles, silicon carbide is desirably used due to itssuperior thermal conductivity.

Furthermore, a pore-forming agent, such as balloons that are fine hollowspheres including an oxide-based ceramic, spherical acrylic particles,and graphite may be added to the sealing material paste, if necessary.The balloons are not particularly limited, and for example, aluminaballoons, glass micro-balloons, shirasu balloons, fly ash balloons (FAballoons), mullite balloons, and the like may be used. Out of these,alumina balloons are more desirably used.

With respect to the material forming the catalyst supporting layer, thematerial having a high specific surface area and capable of highlydispersing the catalyst to support the catalyst thereon is desirablyused, and examples thereof include an oxide ceramic such as alumina,titania, zirconia, and silica. These materials may be used alone, or twoor more kinds of these may be used in combination.

Out of these, the materials having a high specific surface area of about250 m2/g or more is desirably selected, and γ-alumina is particularlydesirable.

Further, the method for forming the catalyst supporting layer made fromabove-mentioned alumina is not particularly limited to the methodexplained in the first embodiment. For example, a method may be used inwhich a honeycomb filter is immersed in a metal compound solutioncontaining aluminum such as an aqueous solution of aluminum nitrate sothat the cell walls are coated with an alumina film through a sol-gelmethod, and the resulting honeycomb filter is dried and fired.

With respect to the catalyst to be supported on the surface of thecatalyst supporting layer, for example, noble metals such as platinum,palladium, and rhodium are desirably used. Out of these, platinum ismore desirably used. Moreover, with respect to other catalysts, alkalimetals such as potassium and sodium, or alkali-earth metals such asbarium may be used. Each of these catalysts may be used alone, or two ormore kinds of these may be used in combination.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A honeycomb filter, comprising: a pillar-shaped honeycomb fired bodyhaving a large number of cells longitudinally disposed in parallel withone another with a cell wall therebetween, with either one end of eachof said cells being sealed, wherein said honeycomb fired body has afirst end face on a gas inlet side and a second end face on a gas outletside such that said honeycomb filter is configured to allow gases toflow in through said gas inlet side and to flow out from said gas outletside, wherein a catalyst supporting layer is formed in acatalyst-supporting-layer area that covers at least about 25% and atmost about 90% of an overall length of said honeycomb fired body andthat abuts said first end face on said gas inlet side, whereinsubstantially no catalyst supporting layer is formed in anon-catalyst-supporting-layer area that covers about 10% of the overalllength of said honeycomb fired body and that abuts said second end faceon said a gas outlet side, wherein a thermal conductivity of saidnon-catalyst-supporting area is larger than a thermal conductivity ofsaid catalyst-supporting-layer area, and wherein gas permeabilitycoefficients k1 (μm²) and k2 (μm²) satisfy the following inequalities(1) and (2):(k1−k2)≦about 0.5  (1), andabout 1.0≦k1≦about 1.5  (2), the gas permeability coefficient k1 being agas permeability coefficient of a cell wall in saidnon-catalyst-supporting-layer area in said honeycomb filter, the gaspermeability coefficient k2 being a gas permeability coefficient of acell wall in said catalyst-supporting-layer area in said honeycombfilter.
 2. The honeycomb filter according to claim 1, wherein a catalystis supported on said catalyst supporting layer.
 3. The honeycomb filteraccording to claim 2, wherein said catalyst comprises a noble metal, analkali metal, or an alkali-earth metal.
 4. The honeycomb filteraccording to claim 1, wherein the thermal conductivity of thenon-catalyst-supporting layer is at least about 1.3 times and at mostabout 5.0 times larger than the thermal conductivity of thecatalyst-supporting-layer area in said honeycomb filter.
 5. Thehoneycomb filter according to claim 1, wherein a main component of saidhoneycomb filter comprises a carbide ceramic, a nitride ceramic, acomplex of a metal and a carbide ceramic, or a complex of a metal and anitride ceramic.
 6. The honeycomb filter according to claim 1, wherein amain component of said honeycomb filter comprises silicon carbide,silicon carbide containing silicon, cordierite, or aluminum titanate. 7.The honeycomb filter according to claim 1, wherein said catalystsupporting layer is provided continuously from the first end face on thegas inlet side.
 8. The honeycomb filter according to claim 1, whereinsaid catalyst supporting layer is provided continuously from a positionapart from the first end face on the gas inlet side.
 9. The honeycombfilter according to claim 1, wherein said catalyst supporting layer isformed on the surface of said cell walls, or is formed inside said cellwalls.
 10. The honeycomb filter according to claim 1, wherein saidhoneycomb filter is formed of a plurality of said honeycomb fired bodieswhich are combined with one another by interposing an adhesive layer, oris formed of a single honeycomb fired body.
 11. The honeycomb filteraccording to claim 10, wherein a coat layer is formed on the outerperiphery of said honeycomb filter.
 12. The honeycomb filter accordingto claim 1, wherein said catalyst supporting layer comprises an oxideceramic.
 13. The honeycomb filter according to claim 12, wherein saidoxide ceramic comprises alumina, titania, zirconia, or silica.
 14. Anexhaust gas purifying apparatus, said apparatus comprising: a honeycombfilter; a casing covering an outside of said honeycomb filter; and aholding sealing material interposed between said honeycomb filter andsaid casing, wherein said honeycomb filter comprises a pillar-shapedhoneycomb fired body having a large number of cells longitudinallydisposed in parallel with one another with a cell wall therebetween,with either one end of each of said cells being sealed, wherein saidhoneycomb fired body has a first end face on a gas inlet side and asecond end face on a gas outlet side such that said honeycomb filter isconfigured to allow gases to flow in through said gas inlet side and toflow out from said gas outlet side, wherein a catalyst supporting layeris formed in a catalyst-supporting-layer area that covers at least about25% and at most about 90% of an overall length of said honeycomb firedbody and that abuts said first end face on said gas inlet side, whereinsubstantially no catalyst supporting layer is formed in anon-catalyst-supporting-layer area that covers about 10% of the overalllength of said honeycomb fired body and that abuts said second end faceon said gas outlet side, wherein a thermal conductivity of saidnon-catalyst-supporting-layer area is larger than a thermal conductivityof said catalyst-supporting-layer area, and wherein gas permeabilitycoefficients k1 (μm²) and k2 (μm²) satisfy the following inequalities(1) and (2):(k1−k2)≦about 0.5  (1), andabout 1.0≦k1≦about 1.5  (2) the gas permeability coefficient k1 being agas permeability coefficient of a cell wall in saidnon-catalyst-supporting-layer area in said honeycomb filter, the gaspermeability coefficient k2 being a gas permeability coefficient of acell wall in said catalyst-supporting-layer area in said honeycombfilter.
 15. The apparatus according to claim 14, wherein a catalyst issupported on said catalyst supporting layer.
 16. The apparatus accordingto claim 15, wherein said catalyst comprises a noble metal, an alkalimetal, or an alkali-earth metal.
 17. The apparatus according to claim14, wherein the thermal conductivity of the non-catalyst-supportinglayer is at least about 1.3 times and at most about 5.0 times largerthan the thermal conductivity of the catalyst-supporting-layer area insaid honeycomb filter.
 18. The apparatus according to claim 14, whereina main component of said honeycomb filter comprises a carbide ceramic, anitride ceramic, a complex of a metal and a carbide ceramic, or acomplex of a metal and a nitride ceramic.
 19. The apparatus according toclaim 18, wherein a main component of said honeycomb filter comprisessilicon carbide, silicon carbide containing silicon, cordierite, oraluminum titanate.
 20. The apparatus according to claim 14, wherein saidcatalyst supporting layer is provided continuously from the first endface on the gas inlet side.
 21. The apparatus according to claim 14,wherein said catalyst supporting layer is provided continuously from aposition apart from the first end face on the gas inlet side.
 22. Theapparatus according to claim 14, wherein said catalyst supporting layeris formed on the surface of said cell walls, or is formed inside saidcell walls.
 23. The apparatus according to claim 14, wherein saidhoneycomb filter is formed of a plurality of said honeycomb fired bodieswhich are combined with one another by interposing an adhesive layer, oris formed of a single honeycomb fired body.
 24. The apparatus accordingto claim 23, wherein a coat layer is formed on the outer periphery ofsaid honeycomb filter.
 25. The apparatus according to claim 14, whereinsaid catalyst supporting layer comprises an oxide ceramic.
 26. Theapparatus according to claim 25, wherein said oxide ceramic comprisesalumina, titania, zirconia, or silica.